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ASE Test Prep: Steering And Suspension


10/25/2006

There will be three questions on the ASE P2 test that deal with the steering and suspension.
 

The steering system gives the driver control over the vehicle’s direction while the suspension allows the tires to roll over bumps and dips in the road without losing their grip. Except for periodic checks of the power steering fluid level, the steering and suspension systems on most vehicles today do not require any maintenance. Parts are not replaced until they wear out or are damaged — unless the vehicle’s owner is upgrading the suspension for a specific purpose such as increased load carrying capacity, towing or handling.

STEERING PARTS

The major components in the steering system include the steering box or rack, inner and outer tie rod ends, idler arms, center links, power steering pump and hoses.Most vehicles today have “rack and pinion” steering that uses a pinion gear on the end of the steering input shaft to move a horizontal bar (rack) sideways. The rack is connected to the tie rods with sockets, which are enclosed in rubber bellows. The linkage has outer tie rod ends only. On some GM applications, a “center-mount” rack is used where the tie rods bolt to the center of the rack rather than the ends. Worn inner sockets can cause steering looseness and tire wear. Wear in a power rack control valve housing can increase steering effort. Fluid inside the bellows indicates leaky seals and a need to replace the rack.

The other type of steering system is the “recirculating ball” steering gear in which ball bearings turn against the worn gear to move the steering linkage. The steering box is connected to the steering linkage with a pitman arm. An idler arm supports the other side of the linkage, which includes a center link, inner and outer tie rod ends and tie rods. Steering wander and looseness can result if the idler arm bushing is worn.

Most vehicles have power steering and use a belt-driven pump to reduce the effort required to steer the wheels. Some power steering pumps also provide hydraulic assist for power brakes (Hydroboost systems). A worn pump will usually make noise and/or leak fluid. If a replacement pump does not come with a pulley, the pulley from the old pump will have to be removed and installed on the new pump. Changing the power steering fluid is also recommended (use the type specified by the vehicle manufacturer).

Power steering systems have two hoses, a high-pressure hose to carry pressure from the pump to the steering gear or power cylinder, and a return hose back to the pump reservoir. Leaks can cause steering problems. Replacement hoses may be preformed or made up using various end fittings and a crimping press.

SUSPENSION PARTS

There are two basic types of front suspensions: short-long arm (SLA) and strut. An SLA suspension uses upper and lower “control arms” of unequal length to support the steering knuckle. Each arm is connected to the knuckle by a “ball joint” (one upper, one lower). A strut suspension typically uses a MacPherson strut in place of the upper control arm and ball joint. The strut combines a shock absorber and spring into one assembly, and serves as the steering pivot for the knuckle. A bearing plate at the top of the strut supports the weight of the vehicle.

On some vehicles, a modified strut configuration is used when the spring is not around the strut, but is located between the lower control arm and subframe. On “wishbone” strut suspensions, the strut supports the weight, but an upper control arm is also used to locate the steering knuckle. Almost all front-wheel drive vehicles, as well as many rear-wheel drive vehicles, have strut suspensions. SLA suspensions are used on most rear-wheel drive cars and light trucks.

Suspension parts that may need to be replaced include ball joints, shocks, struts, control arm bushings and springs. Most people don’t think springs ever wear out, but over time the constant force of gravity can cause springs to weaken and sag. This changes ride height and alignment, which can increase tire wear and cause handling and ride problems.

Shocks and struts are the most commonly replaced items because they suffer the most wear. Inside is a piston that pumps back and forth through an oil-filled tube. This creates friction that dampens the motions of the suspension and keeps the vehicle stable and the tires in contact with the road. Control valves in the piston and the bottom of the shock or strut vary the resistance by venting fluid as the velocity of the piston changes. The piston rod has a seal that keeps the oil inside and prevents outside contaminants from entering the shock or strut. Over time, this seal wears out and allows the precious fluid inside to leak out. As a result, the shock or strut loses its ability to control the suspension, ride quality goes out the window and the suspension becomes bouncy and rough.

There are two basic types of shocks and struts: twin-tube and monotube. Twin-tube shocks have an oil reservoir around the outside of the piston chamber. Oil moves back and forth from the chamber through the valves in the end of the shock. Monotube shocks are only a single tube with no outer chamber. One end of the shock is filled with pressurized gas with a floating piston seal separating the gas charge from the oil. Twin-tube shocks may also be pressurized with nitrogen gas because gas-charging reduces cavitation, foaming and shock fade. Monotube shocks are typically charged at a much higher pressure (up to 360 psi) and are used more on performance applications because of their quick-acting and firmer ride characteristics.

Shocks and struts are usually replaced in pairs or sets. Replacement shocks and struts with larger piston bores, increased gas pressure or other special features like adjustable valving can be installed to upgrade ride control performance. Likewise, monotube shocks are often used to replace twin-tube shocks to increase suspension stiffness and cornering agility.

One item that may also need to be replaced when replacing struts are the “bearing plates” atop the strut that allow the strut housing to pivot when the wheels are steered. Looseness in the bearing plate can cause steering noise. Binding may increase steering effort and prevent the wheels from recentering following a turn.

MacPherson strut assemblies usually require a spring compressor to disassemble and reassemble, but preassembled replacement struts with new springs and bearing plates are also available to make installation easier. Wheel alignment is usually necessary after replacing struts.

Ball joints are another part that may need to be replaced at some point in the vehicle’s life. The joints connect the steering knuckle to the control arms. They may also be used on rear control arms. Joints that carry weight are called “loaded joints” while those that do not are called “follower” joints. SLA suspensions have two upper and two lower ball joints. MacPherson strut suspensions have only two lower ball joints.

A ball joint is so named because of its ball-and-socket construction. The ball stud may ride against a metal gusher bearing, or it may be highly polished to reduce friction and be enclosed in a polymer bearing. Most low-friction ball joints are sealed and do not have a grease fitting for lubrication. Older, gusher style joints have grease fittings so they can be lubricated periodically.

When ball joints become worn, they can make suspension noise and upset wheel alignment. There is also a danger the joint may separate, allowing the suspension to collapse. Replacing a ball joint requires a separator tool or fork to separate the stud from the knuckle once the stud nut has been removed. Some joints are bolted or riveted to the control arm, others are screwed in and others are press-fit into the arm.

  Previous Comments
avatar   dsinferno   star   9/24/2009   3:41 PM

Thank you for this page it was just what I was looking for and needed. It answered all my questions

















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